1. Field of the Invention
The present invention relates generally to an image forming apparatus.
2. Description of the Related Art
Conventionally, various techniques have been devised to maintain image density and tone reproduction in image forming apparatuses. For example, Japanese Patent No. 4016949 and Japanese Patent Application Laid-open No. 2005-128484 disclose techniques of controlling toner-image forming conditions. In this technique, information about the amount of toner that has stuck to a toner patch formed on an image carrier is obtained and toner-image forming conditions are changed based on the obtained toner amount.
A typical example of means for obtaining a toner amount is a photosensor. More specifically, an amount of toner per unit area of a toner patch, or a toner image, is calculated based on a result of detection of the toner patch by the photosensor. More particularly, the mass of toner per unit area of the toner image (hereinafter, “M/A”) is calculated from the detected amount of toner per unit area of the toner image on an assumption that particle sizes of the toner that forms the toner patch have a specific distribution.
However, when actual particle size distribution differs from the specific distribution, an error occurs in the calculation of an M/A, making it difficult to obtain an accurate value of M/A.
Meanwhile, because rupture of toner particles can occur during repeated use of the toner particles in the development process, particle size distribution of toner can change over time. A developing unit generally has a characteristic, what is called a selective development, that toner particles in a specific particle size range are likely to be selectively used in development. Due to this selective development, particle size distribution of toner changes over time because toner particles in the specific particle size range are consumed earlier than other toner particles. Developing units can be broadly classified in process-cartridge type developing units and toner-replenishing type developing unit. In the process-cartridge type developing unit, the developing unit is replaced. On the other hand, in the toner-replenishing type developing unit, only the toner is replenished. The change in particle size distribution of toner over time is greater in the process-cartridge type developing units than in the toner-replenishing type developing units. The reason is that, when toner that has been initially filled in the process cartridge is used up, the developing unit is replaced rather than supplying toner from outside. Accordingly, in an image forming apparatus that includes a process-cartridge type developing unit, the disadvantage that it is difficult to obtain an accurate value of toner amount due to the change in particle size distribution of toner is pronounced.